Keilim, Chapter 24, Mishnah 12. Our mishnah continues the chapter's pattern of taking a single type of object and dividing it into three, showing how the very same material can fall into three completely different categories of tumah depending on what a person has made of it. Here the object is an animal hide.
The wording of the mishnah is "Shelosha orot hen": when it comes to tumah, leather divides into three separate categories.
A hide made to be spread out
The first case: "Ha'asui lishtiach", a hide that was prepared to be laid out flat as a mat or rug, so that people can sit on it. Such a hide is "tamei midras". Since it was designated for sitting, it belongs to the family of objects that serve a person's body, and it can therefore receive midras tumah, the tumah conveyed by the pressure of a zav or the like sitting or treading upon it.
A hide used to wrap utensils
The second case: "Litachrich hakelim", a hide used as a covering in which utensils are wrapped, a kind of protective sheath for them. This hide is "tamei tumat met", it can receive tumah from a corpse, but not midras tumah.
Why the difference? It is not made for a person to sit upon, so midras does not apply to it. And although in its natural state it is simply a flat piece of leather with no cavity at all, it does not remain flat in use: it is folded around the utensils in order to hold them together and keep them in place. That folding turns it into a functioning container, and a container can become tamei with the tumah of a corpse.
A hide set aside for straps and sandals
The third case: "veshel retzuot veshel sandalim", a hide that has been designated to be cut into straps or made into sandals. The mishnah rules "tehorah mikelum", it is pure from every form of tumah.
The reason is that this hide is not yet anything at all. It is raw material awaiting the work that will turn it into straps or into footwear, and until that work is done it is an unfinished utensil. An unfinished utensil is not susceptible to tumah of any kind.
Three identical pieces of leather, then, and three different halachot: one is a seat and receives midras, one is a wrapper and receives corpse tumah alone, and one is still only a plan for a utensil and receives nothing.